Admittedly it is a bit of a broad question and difficult to answer without seeing you hit a ball. If the ball keeps flying then the least of your worries is bending thee knees. The most important parts of any groundstroke are hitting the ball in front of you and having a good follow through. That doesn't mean following through for the hell of it - it means that you have a good follow through as a result of a good stroke action and racket speed. Apart from getting a good coach the next best thing you could do is ask a good player to show you what they do and what grip they use = if the ball is flying up in the air you might want to close the face a bit by turning your grip further round to the right. Another good thing to do is to go onto youtube and type in "forehand slow motion" and watch a few of the pros in action and copyi their movements. Concentrate on just hitting the ball first and when you can do that consistently you can add the things that make the stroke better like knee bend, rotation of trunk. footwork etc. And hit the ball low to high to keep it in with topspin.

Track the incoming ball with your racket out in front of you until you see the ball bounce then take the racket back slightly; point the strings down parallel to the ground (the racket will open up automatically as you swing); approach the ball with the edge of your racket slowly with a loose grip, finding the ball with your hand and step to the outside (not forward), swinging from low to high and pulling across your body using your bicep and pectorals (not out in front of you), and finish the stroke over your shoulder, pointing the butt of the racket toward the intended target. You can even pull back towards the fence as you come across and finish. These are exaggerated moves which will help you grasp the essence of the modern fh used by all today's top players.